The present invention refers to a DC/AC converter, that is a device able to convert a DC generator into an AC one, in particular the type having galvanic insulation between input and output.
The subject of the invention can be employed in a wide range of applications, among which the one of ringing generator for telephone systems, as emergency supply for computer systems or as driving stage for AC motor.
To convert direct current, e.g. supplied by a battery, into alternate current (preferably sinusoidal) several topologies are known. Some topologies have several disadvantages, such as for instance considerable weight and overall dimensions, or a high output distortion.
In particular, the most advanced DC/AC converter includes a high frequency PWM converter stage with galvanic insulation which is controlled by a sinusoidal voltage. It also includes a compensation network and a transistorized full-bridge. In this converter, the first stage does not generate a direct voltage, but a rectified sinewave. The second stage (full-bridge) allows to extract the sinewave and working at low frequency it is able to get high efficiency. Unfortunately this converter requires in practice two stages and an additional compensation network to allow proper operation on reactive load.
Another DC/AC topology uses a full bridge PWM stage followed by a low frequency transformer, but the efficiency figure obtainable at relatively low input voltage is poor.
WO 82/02134 discloses a DC/AC converter suitable for producing an alternating output voltage on a load in accordance with the preamble of claim 1. However according to such prior art device a sinusoidal waveform is reconstructed in four time intervals under control of bidirectional switches that connect the converter branches one after the other in correspondence of the zero-current crossing. This device requires complex control means since a pair of precisely timed control signals is requested for each bidirectional switch to prevent discontinuities in the generated waveform, and moreover such timing is dependent from the load.